It is well known that primarily two types of herbicides are in use. The first type is selective and kills only "target" plants (weeds). This type is applied generally over the treatment area, such as by spraying. The second type is nonselective and kills almost all plants it contacts. Usually, only a small quantity of the second type, applied to the leaves or foliage, is needed to effectuate the "kill".
It is obvious that widespread spraying of a selective herbicide can be expensive (and inefficient), for a substantial portion of the herbicide fails to reach the "target". The herbicide that is applied to the desired crop and bare ground is wasted. Another problem with selective herbicides is that they are not always effective, as there are not selective herbicides for all crops and all weeds.
Nonselective herbicides are much more efficient and thus less expensive to use. Such herbicides are not applied by spraying, for contact with the desired crop would kill it. This is why considerable effort has gone into finding a means for selectively applying a nonselective herbicide. There is no waste when the herbicide is applied only to the plants intended to be killed.
One general characteristic of weed growth that has made the selective application of a nonselective herbicide practical is the propensity of weeds to quickly grow taller than the desired crops. Thus, early in the growing season when the crops are short and the weeds much taller, a direct application of herbicide to the foliage of the weeds by means of wiping can be accomplished with good results.